This invention relates to a method and an apparatus for crystallization of a solution containing salts, and more particularly to a method and an apparatus for crystallizing salts as crystals by concentrating a solution containing salts by heating and then cooling the solution.
Recently a large amount of industrial waste water is discharged from factories owing to development and expansion of the industry, and soluble salts are contained in the industrial waste water. It is necessary to remove these salts to prevent environmental pollution.
The salts are usually removed from solutions as crystals, which method is called "crystallization of salts", wherein the property that the solubility of salt changes with a change in temperature of the solution is utilized, but deposition of scales on a heat transfer surface is a serious problem.
Japanese Patent Publication No. 16161/66 discloses an apparatus for crystallization of a solution containing salts, which has a concentrator for generating crystal nuclei, and a crystallizer for making the crystal nuclei grow into much larger crystals.
Japanese Patent Publication No. 5007/64 discloses an apparatus for concentrating a solution by indirect heating and vacuum evaporation. In the case of indirect heating, a heat exchanger is utilized, wherein a heat exchange is carried out between a solution containing salts which passes through the inside of the pipe and a hot heating medium which passes along the outside of the pipe, and thus crystals of salts are liable to deposit as scales on the inside wall of the pipe at the pipe outlet of the heat exchanger. In that case, the heat transfer area is reduced by the deposition of scales, resulting in decrease in heat transfer efficiency or deterioration in passage of the solution through the inside of the pipe. That is, the apparatus is sometimes damaged by increased pressure of the solution.
In the conventional crystallizer, the inside of the crystallizer is kept in vacuum or the solution merely left standing for settling, as shown in said Japanese Patent Publication No. 5007/64, and thus it takes inevitably much time in crystal growth. This is because the conventional crystallizer is directed to production of crystals with a uniform quality and size. However, in the crystallizer for the waste water treatment, it is necessary to remove a large amount of crystals from the solution within a short time. That is, it is necessary to cool the solution in the crystallizer uniformly within a short time.